1. Field of the Invention
The present invention relates to a semiconductor device and a semiconductor module, and particularly, relates to a lead frame type semiconductor device and semiconductor module in which a comparatively large-sized semiconductor element is sealed with a resin. The present invention further relates to a method for manufacturing a semiconductor device, and to a lead frame, and particularly relates to a method for manufacturing a semiconductor device and a lead frame in which a lower surface of an island on which a large-sized discrete type semiconductor element is mounted is sealed with a resin.
2. Description of the Related Art
Semiconductor devices in which a semiconductor element that configures a power supply circuit and the like is sealed with a resin have been developed.
With reference to FIGS. 17A and 17B, a configuration of a semiconductor device 100 that is of this kind will be described. FIG. 17A is a plan view of the semiconductor device 100, and FIG. 17B is a sectional view taken along the line B-B′ in FIG. 17A.
As shown in FIGS. 17A and 17B, the semiconductor device 100 includes a semiconductor element 104, an island 102 on which the semiconductor element 104 is mounted, lead 110s that are connected to the semiconductor element 104 and partially project to the outside, and an sealing resin 108 that covers and seals these components in an integrated manner.
The semiconductor element 104 is, for example, a discrete MOSFET. A drain electrode on a lower surface of the semiconductor element 104 is connected to the island 102, a gate electrode on a front surface thereof is connected to a lead 110A through a metal thin wire 106, and a source electrode on the front surface thereof is connected to a lead 110C through the metal thin wire 106.
Moreover, the leads 110A to 110C project from a side surface of the sealing resin 108. These leads 110 are inserted into a mounting board so that the semiconductor device 100 is fitted and mounted.
A method for manufacturing the semiconductor device 110 having the above-mentioned configuration is as follows. First, etching processing or press processing is performed on a conductive plate made of copper or the like and having a thickness of approximately 0.6 mm to form the island 102 and the leads 110 each having a predetermined shape. Then, the semiconductor element 104 is fixed on a front surface of the island 102, and electrodes on a front surface of the semiconductor element 104 are connected to the leads 110A and 110B through the metal thin wires 106. Thereafter, the island 102 and the semiconductor element 104 are accommodated in a cavity of a mold, and injection molding is performed by filling the cavity with the sealing resin 108. The semiconductor device 100 is manufactured in such a process.
Moreover, the above-mentioned process is performed with the leads 110 and the island 102 being connected to each other by a frame-like lead frame.
When the so-called power element in which switching is performed for large current is employed as the semiconductor element 104, a large amount of heat is discharged from the semiconductor element 104. In order to prevent the semiconductor device 100 from overheating due to this heat, a lower surface of the sealing resin 108 shown in FIG. 17B is brought into contact with a heat sink. In this case, a front surface of the sealing resin 108 is pressed downward by a pressing member such as a screw or the like, so that the lower surface of the sealing resin 108 is thermally coupled to the heat sink.
However, the above-mentioned semiconductor device 100 has a problem that the semiconductor element 104 could be destroyed by a pressing force of the screw that brings the semiconductor device 100 into contact with the heat sink.
Specifically, with reference to FIG. 17A, in order to achieve the semiconductor element 104 having low on resistance and high breakdown voltage, increasing the size of the semiconductor element 104 in a plane is effective. On the other hand, in order to implement miniaturization and weight reduction of the semiconductor device 100, a reduction in size the sealing resin 108 and the island 102 in planes is preferable. For that reason, the size of the semiconductor element 104 in the plane is a little smaller than those of the sealing resin 108 and the island 102. Namely, a portion used as an area for mounting the semiconductor element 104 accounts for a large percentage of the whole area of the semiconductor device 100.
Accordingly, when the front surface of the sealing resin 108 shown in FIG. 17B is pressed downward to bring the lower surface of the semiconductor device 100 into contact with a heat dissipating member such as a heat sink, the semiconductor element 104 is disposed under an area thus pressed. In other words, an area of the sealing resin 108 pressed by pressing member superimposes the area on which the semiconductor element 104 is placed. Therefore, this pressing force may apply large stress on the semiconductor element 104, resulting in breakage of the semiconductor element 104.
In addition, when a thickness of the sealing resin 108 that covers the semiconductor element 104 is made thinner in order to obtain a low profile semiconductor device, the above-mentioned problem is more likely to occur.
Furthermore, the above-mentioned method for manufacturing the semiconductor device has a problem that the lower surface of the island 102 is not sufficiently covered with the sealing resin 108.
Specifically, with reference to FIG. 17B, in order to efficiently discharge heat caused from the semiconductor element 104 to the outside while maintaining insulation of the island 102, it is important to make the sealing resin 108 to thinly cover the lower surface of the island 102.
However, as described above, the sealing resin 108 is formed by injection forming by use of a mold. Accordingly, if the sealing resin 108 for covering the lower surface of the island 102 is approximately 0.4 mm, for example, a space between the island 102 and an inner wall of the mold becomes very narrow, therefore making it very difficult to cause a liquefied sealing resin 108 to flow into this space. Accordingly, the space between the lower surface of the island 102 and the mold is not sufficiently filled with the sealing resin 108, leading to a problem that the island 102 is partially exposed to the outside from the sealing resin 108. Moreover, when a resin material filled with a particulate filler is employed as the sealing resin 108 in order to have improved heat dissipation properties, viscosity of the sealing resin 108 may increase, and the above-mentioned problem may frequently occur. Furthermore, when a large-sized discrete type transistor having low on resistance and high breakdown resistance is employed as the semiconductor element 104, an area of the island 102 also increases. Accordingly, there arises a problem that it is difficult to sufficiently cover the lower surface of the island 102 with the sealing resin 108.